1. Field of the Invention
The invention relates to a method and a device for applying photoresist to a base body surface.
From semiconductor coating, especially of wafers, methods such as spinning, immersion or spraying are known for applying a coating of photoresist. Because an exposure time has to be selected, especially in the coating of wafers, a precise resist thickness is very important.
In the spinning method, for example, the wafer is rotated after the photoresist has been applied as centrally as possible thereon. The centrifugal forces that are then operative thus distribute the photoresist substantially uniformly over the surface of the flat wafer.
Immersion and spray methods can be employed, for example, for three-dimensional substrates. In the immersion method, however, it often happens that the layer thickness cannot be made homogeneous in all the regions of the surface, but rather, a bead of resist remains at the end of the surface region and does not permit uniform exposure or illumination. A problem also arises in the, spraying method. Although relatively large surfaces may be wetted quite well therewith, nevertheless, the resist contracts during drying. As a result, similarly to the immersion method, deviations occur in the peripheral region of the structure which impair the uniformity of the resist layer. The thickness of the resist beads at the edge can amount to five times the desired layer thickness.
The application of photoresist to smooth base bodies is employed for photolithographic techniques. These techniques are used to produce layer structures, with the aid of masks or locally acting beam-type recording devices using high-energy radiation, a radiation-sensitive material being applied to a surface to be structured and irradiated, and then developed. In this way, photoresist patterns that serve as a mask for various thin-film technologies can be created. Irradiating the radiation-sensitive material can be performed, for example, by electromagnetic waves or corpuscular radiation.
In photolithography, ultraviolet light having a wavelength of approximately 430 nm to 200 nm is typically employed. Photolithographic methods are widely applicable in the field of producing semiconductor circuits, sensors, or microcircuitry systems.
In the heretofore known applications, even and, in particular, macroscopically flat substrates, or substrates with only very slightly curved surfaces, are coated as base bodies.
In the conventional spinning method, the desired photoresist thickness is adjusted by way of the rotational speed of the spinner and the viscosity of the photoresist. Depending upon the particular application, layer thicknesses of a few hundred nanometers to a few tens of micrometers are needed, and must be applied with great homogeneity in the region of the surface of the base body to be structured. As already noted hereinbefore, however, the spinning method can be used only for macroscopically even or flat surfaces of a base body.
The beads of resist that form as a resist-thickness inhomogeneity in the peripheral region, particularly in the immersion method, lead to an impermissible spacing between the mask and the substrate known as the "proximity effect", upon the exposure to light in the contact method. This affects the quality and resolution of the photoresist structures, which proves to be disadvantageous. A further disadvantage is considered to be that this peripheral region, because of the greater layer thickness, receives too slight a radiation dosage, and the patterns provided thereat are therefore only inadequately formed, and/or residues of resist remain between the patterns.
With resist coating by the spray method, the same structural problems discussed above arise. Furthermore, with this method, the layer thicknesses of a few micrometers that are typically employed for photolithography can be attained only with highly diluted photoresists, which has a disadvantageous effect upon the quality of the photoresist layer. The layer has inhomogeneities in resist thickness at the corners and edges. In addition, with the spraying method, only large-area coatings can be provided, which, from a technological and economic standpoint, with a view to low photoresist consumption, is actually undesirable. In view of these considerations, it would be more appropriate to coat defined portions of a surface. Another disadvantage of both the spraying method and the immersion method is the comparatively high consumption of photoresist.
A method of spraying layers of flat or even surfaces has become known heretofore from the published European Patent Document EP 0 609 478 A1. In this regard, discrete drops are applied to the surface and are then evened or spread out evenly in further method steps. The method is also suitable only for flat substrates, such as printed circuit boards.
The published German Patent Document DE 43 29 338 A1 discloses only a flat substrate in which a pattern is produced by a type of ink jet printing head.
In the prior art, to apply relatively thick layers of photoresist, photoresist films are applied to base bodies, as a result of which better homogeneities in thickness can be achieved. However, these techniques are suitable only for relatively great layer thicknesses. Furthermore, when used for base bodies in the form of bodies of rotation, a seam is formed.
From the published German Patent Document DE 30 12 988 A1, a device and a method for producing a printing plate blank have become known, wherein a plate with a substrate is provided with a photopolymer strip of precisely dimensioned shape and thickness. To that end, the device is provided with a reservoir for the photopolymer, a bar or beam movable across the plate and being formed with a channel at an underside thereof, supply lines connecting the channel to the reservoir, and a stripper blade formed in the bar. During the production of the printing plate blank, the photopolymer layer is applied to the substrate. Next, the photopolymer layer is irradiated through a mask by a chemically active radiation, and the nonirradiated region covered by the mask is then removed. The photopolymer is delivered under the influence of gravity and, to that end, the photopolymer is kept at a pressure of a predetermined liquid level of 2.5 to 25 cm in the reservoir. The viscosity of the photopolymer is described in this reference as being 500 to 20,000 cP, preferably 2000 cP. The photopolymer layer is applied with a layer thickness of 25 to 510 .mu.m. The supply lines for the liquid photopolymer are provided as capillary channels formed in the bar. The individual supply lines have a diameter of 0.76 to 12.7 mm, in particular 2.28 to 6.35 mm. The individual outlet openings and supply lines, respectively, have a mean spacing of 3.18 to 25.4 mm, and in particular 6.35 to 12.7 mm. As a result, a polymer strip approximately 40 cm wide and approximately 60 cm long is formed in one operating step or pass on the substrate of the smooth plate of the printing plate blank.